Optical receiver, optical transceiver using the same, and control method of reception of optical signals

ABSTRACT

An optical receiver includes photodetectors each provided on a corresponding channel of multiple channels and configured to detect an optical signal at the corresponding channel, a delay adjuster circuit provided before the photodetectors and configured to adjust a delay time of an optical waveform of an incoming signal on at least one channel, and a processor that controls the delay time such that a change point of a first optical waveform of a first channel is away from a data decision timing of a second optical waveform of a second channel, using information acquired from an electrical signal produced after photo-detection.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT International Application No.PCT/JP2016/066837 filed on Jun. 7, 2016 and designating the UnitedStates, the entire contents of which are incorporated herein byreference.

FIELD

The embodiments discussed herein relate to an optical receiver, anoptical transceiver using the same, and a control method of reception ofoptical signals.

BACKGROUND

Today, to achieve high-capacity optical communications, development ofmultichannel optical transceivers is in progress. For example, atransmission scheme using four 25-Gpbs channels in combination withpulse amplitude modulation is adopted in optical transceivers designedfor 100 gigabit Ethernet.

A receiver optical subassembly (ROSA) is provided in the receiverfrontend of an optical transceiver for optical-to-electrical conversionand amplification. To reduce the ROSA size, a compact design forincorporating a plurality of channels into a package is being adopted.

In multimode transmission using discrete photodetectors, independentlyadjustable optical delay devices are arranged on single-mode fiber opticcables connected to the associated photodetectors. The optical delaydevices are controlled by a digital signal processor (DSP) to bring allthe incoming light signals in synchronization with one another. See, forexample, Patent Document 1 listed below.

Another known technique is an electronic circuit for use in paralleltransmission of electrical signals, adapted to correct for or retime askew or a difference in propagation delay time between the clock andsynchronized parallel data. See, for example, Patent Document 2 listedbelow.

RELATED-ART DOCUMENTS Patent Documents

Patent Document 1: PCT International Publication No. WO 2012/150127

Patent Document 2: US Patent Application Publication No. 2004/0125902 A1

SUMMARY

According to an aspect of an embodiment, an optical receiver includes

photodetectors each provided on a corresponding channel and configuredto detect an optical signal at the corresponding channel,

a delay adjuster circuit provided before the photodetectors andconfigured to adjust a delay time of an optical waveform of an incomingsignal on at least one channel, and

a processor that controls the delay time such that a change point of afirst optical waveform of a first channel is away from a data decisiontiming of a second optical waveform of a second channel, usinginformation acquired from an electrical signal produced afterphoto-detection.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

Additional objects and advantages of the embodiments will be set forthin part in the description which follows, and in part will be obviousfrom the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates influence of crosstalk between channels;

FIG. 2 illustrates influence of crosstalk between channels;

FIG. 3 illustrates a configuration that is not affected by crosstalk;

FIG. 4 is a schematic diagram of an optical transceiver with a monitorsignal fed back from a block positioned after the ROSA;

FIG. 5 illustrates a feedback control architecture according to thefirst embodiment;

FIG. 6 illustrates a feedback control architecture according to thefirst embodiment;

FIG. 7 illustrates a configuration example of the feedback controlaccording to the first embodiment;

FIG. 8 is a flowchart of signal reception control operations accordingto the first embodiment;

FIG. 9 illustrates a configuration example of feedback control accordingto the second embodiment;

FIG. 10 is a flowchart of signal reception control operations accordingto the second embodiment;

FIG. 11 is a flowchart of the control routine of a group delay variableelement in step S24 of FIG. 10; and

FIG. 12 illustrates an advantageous effect of group delay control onoptical waveforms.

DESCRIPTION OF EMBODIMENTS

Prior to describing the details of configuration examples and processesof the embodiments, explanation is made with respect to influence ofcrosstalk arising in a compact ROSA package with reference to FIG. 1 andFIG. 2. In a ROSA package with multiple channels packed into a singleunit, the receiver sensitivity is degraded due to crosstalk betweenchannels. Unlike a conventional configuration in which light receivingsections are separated from one another between channels, inter-channelinterference becomes conspicuous in a compactly integrated ROSAconfiguration. In particular, when optical signals are input to the ROSAwith the data change points of the optical waveforms of incoming signalsoffset from one another, crosstalk occurs between channels due tovoltage swing of the electrical signals produced afteroptical-to-electric conversion is performed.

For example, in optical transmission using n+1 channels (from Ch-0 toCh-n) as illustrated in FIG. 1, the change points of the opticalwaveforms received at the ROSA may not be synchronized with each otherdepending on channels. Even if multichannel optical signals aretransmitted from a transceiver in such a manner that the optical phasesof the transmission signals are synchronized with one another, thechange points of the optical waveforms are hardly maintained insynchronization with one another due to environmental influence ofoptical transmission such as variation in optical path lengths, phasefluctuation in optical waveforms of the transmitted signals, etc. Evenif the physical lengths of the optical transmission paths are designedto be the same, the effective propagation length and/or the phaserotation may change depending on a certain factor such as the index ofrefraction. Moreover, the data change points of the optical waveforms ofmultichannel signals may already be out of synchronization betweenchannels at the transmitter frontend. In such cases, a time lag isgenerated between change points of optical waveforms when input to aROSA.

When the ROSA unit receives multichannel optical signals with datachange points of the optical waveforms offset from one another,crosstalk will occur between transimpedance amplifiers (TIAs). Each ofthe incoming optical signals is detected as a photocurrent by theassociated photodiode (PD), and the photocurrent is converted into avoltage signal by the associated TIA. At this point of time, the voltageswings with a predetermined amplitude at the rising edge and the fallingedge of the transmitted data. The electromagnetic wave generated by thevoltage swing cuts into or interferes to other channels inside the ROSAunit and eventually crosstalk occurs. Crosstalk is undesirable becausethe data decision accuracy of a clock and data recovery (CDR) circuitdecreases and the receiver sensitivity is degraded.

FIG. 2 illustrates adverse influence of crosstalk occurring when thedata change points of optical waveforms of incoming signals are out ofsynchronization. In the top half of FIG. 2, the data change point of theoptical waveform of Channel-1 is offset from the data change point ofthe optical waveform of Channel-2. The bottom half of FIG. 2 illustratesinfluence on the electrical waveform of Channel-2 from Channel-1. Thedecision threshold used for data decision (i.e., determination oftransmitted data bit values) is set to, for example, the middle of thevoltage swing. The phase of decision which indicates a timing for datadecision is set in or near the middle of two adjacent change points.

If a time difference in data change point is not negligible betweenchannels as illustrated in the top of FIG. 2, voltage swing at the datachange point of the optical waveform of Channel-1 will influence on theChannel-2 input signal. This interference causes the margin for datadecision to narrow at the phase of decision as illustrated in the bottomof FIG. 2, and wrong decision may be made during data decision(determining bit “1” or bit “0”). The same adverse influence occurs fromChannel-2 to Channel-1, and degrades the receiver sensitivity ofChannel-1. Crosstalk may occur not only between Channel-1 and Channel-2,but also among all the channels incorporated in the ROSA package.

FIG. 3 illustrates a scenario where the data change points of opticalwaveforms of the incoming signals are time-aligned between Channel-1 andChannel-2. With an optical frontend circuit with photodiodes and TIAsintegrated in a compact ROSA package, the change point of an electricalsignal (i.e., voltage swing) of one channel may still influence on otherchannels. However, by controlling such that interference from a certainchannel occurs at or near the change point of other channels, adverseeffects on data decision can be avoided. This is the basic idea of theinvention and illustrated at the bottom of FIG. 3, in which interferencefrom Channel-1 occurs at the change point of the electrical waveform ofthe Channel-2 received signal. In this case, the crosstalk rarelyinfluences on the threshold decision at the phase of decision.

Techniques disclosed in the embodiments reduce inter-channel crosstalkby realizing the configuration of FIG. 3. Namely, the data change pointsof the optical waveforms are time-aligned at the input end to a ROSA,using information acquired from a block subsequent to photo detection(e.g., by photodiodes).

FIG. 4 is a schematic diagram of an optical transceiver 1 according toan embodiment. The optical transceiver 1 includes an optical receiver 10and an optical transmitter 20. At the receiving end, optical signals arereceived at the optical receiver 10 from the optical transmission path30R, and electrical signals are output from the optical receiver 10. Atthe transmission end, electrical signals are input to the opticaltransmitter 20 and optical signals are output to the opticaltransmission path 30T.

In the optical transmitter 20, the input electrical signal is subjectedto waveform shaping and retiming processes at CDR 25, and then convertedinto high-speed drive signals at drivers 24-0 through 24-n provided onthe respective channels. The high-speed drive signals are supplied to atransmitter optical subassembly (TOSA) 23. TOSA is an integratedfrontend module on the transmitter side. Since the configuration and theoperations of the TOSA unit are not directly related to the presentinvention, detailed explanation for it is omitted here. Briefly, opticalmodulators and a laser diode serving as a light source are incorporatedin the TOSA unit, and the optical modulators are driven by the drivesignals at the respective channels. The optical modulators may beelectro-absorption modulators (semiconductor devices) or lithium niobate(LN) modulators. The optical signals generated on the respectivechannels are multiplexed at the optical multiplexer 25, and output tothe optical transmission path 30T.

In the optical receiver 10, multichannel optical signals received fromthe optical transmission path 30R are demultiplexed at the opticaldemultiplexer 16 into optical signals of the individual channels. Themultichannel optical signal may be a wavelength division multiplexed(WDM) signal, a space division multiplexed signal, or a mode divisionmultiplexed signal.

On the input side of the ROSA 13 is provided a delay adjuster circuit 11configured to adjust the delay time of the optical signal input to theROSA 13 on each channel. Delay time on each channel can be adjusted by,for example, minimizing the group delay. In the delay adjuster circuit11, the delay time of the optical signal propagating through theindividual channel is variable such that the data change points of thewaveforms of the incoming signals are time-aligned with one anotherbetween channels. Control operations for the time-alignment of the datachange points are performed using information from electrical signalsobtained after the photo-detection of the optical signals at thephotodiodes 131-0 to 131-n, which will be described in more detailbelow.

The delay adjusted optical signals of the respective channels aresupplied from the delay adjuster circuit 11 to the ROSA 13, andconverted into photocurrents by the photodiodes 131-0 to 131-n. Thephotocurrents are then converted into electric voltages by TIA 132-0 to132-n. The voltage signals output from the ROSA 13 are supplied to CDR15. A set of linear amplifiers may be provided inside the ROSA 13, orbetween the ROSA 13 and the CDR 15, to amplify the voltage levels of theoutput signals of the TIA 132-0 to 132-n to an appropriate level.

The CDR 15 extracts or generates a clock from each of the channels,restores the data signal using the clock, and outputs an analogelectrical signal. The analog electrical signals of the respectivechannels are converted into digital signals, and subjected to thesubsequent processes including channel estimation, chromatic dispersioncompensation, polarization processing, error correction, etc. in adigital signal processor (DSP). Thus, the transmitted data signals aredemodulated and decoded.

Information indicated by the electrical signals obtained after thephoto-detection at the photodiodes 131-0 to 131-n is supplied via thecontroller 17 to the delay adjuster circuit 11, as indicated by thedashed line in FIG. 4. Delay time on each channel is adjusted by thedelay adjuster circuit 11 under the control of the controller 17. Theinformation represented by the electrical signal includes, but is notlimited to, clocks extracted at the CDR15, signal receive qualityinformation such as a bit error rate, and so on. The electric voltagesoutput, from the TIAs on the respective channels may also be used.

Detailed explanation is now made to particular configurations andtechniques for group delay control.

<First Embodiment>

FIG. 5 and FIG. 6 illustrate examples of group delay control performedin the optical receiver 10A of the first embodiment. In FIG. 5, afterchannel separation and before photo-detection at the photodiode 131-0 to131-n, a group delay adjuster 11A (which is an example of the delayadjuster circuit 11) is provided to adjust the delay time on therespective channels. The group delay adjuster 11A includes group delayvariable elements 112-0 to 112-n provided for the respective channels.

Group delay is a parameter representing the delay time of a propagatingoptical waveform. Group delay is different from phase delay. Phase delayrepresents the absolute value of a delay time calculated by dividing aphase difference (φ) between input and output waveforms by angularfrequency (ω). In contrast, group delay is a derivative of a phasedifference with respect to angular frequency and it represents a rate ofchange of the optical phase with respect to frequency. By minimizing thegroup delay (i.e., rate of phase shifting), the delay time can bemaintained constant.

The group delay variable elements 112-0 to 112-n are made of, forexample, silicon (Si), a polymer, an electro-optic material, a photoniccrystal, etc. With silicon or polymer, the index of refraction can bechanged by temperature control. With electro-optic materials or photoniccrystals, the index of refraction can be changed by controlling avoltage to be applied thereto. As the index of refraction changes, thepropagation speed of light wave travelling through the medium varies,and accordingly, group delay or delay time can be adjusted.

As illustrated in FIG. 5, if a time difference exists between channelsat the data change point of the optical waveform, then crosstalk occursamong TIA 132-0 to 132-n. The waveforms of the electrical signals to beinput to the CDR 15 are distorted, and clock extraction and datadecision at the CDR 15 become difficult. Although FIG. 5 illustrates atime difference at the data change points between channel-1 andchannel-2 for the sake of convenience of illustration, such timedifferences at data change points may occur across the channels.

The electrical waveforms output from the TIA 132-0 to 132-n directlyreflect the time differences at the change points of the associatedoptical waveforms input to the ROSA 13. Accordingly, in the firstembodiment, the change point of the incoming optical signal on each ofthe multiple channels is detected based upon the electrical signalwaveform output from associated one of the TIA 132-0 to 132-n. Thedetection result is fed back to the group delay adjuster 11A to minimizethe time difference at the data change points among the multiplechannels.

FIG. 6 illustrates an example of group delay control configuration. Forexample, channel-1 is controlled so as to decrease the propagation speedof the optical signal travelling on that channel, while channel-2 iscontrolled so as to increase the propagation speed of the optical signaltravelling on that channel. Under this control, the change points of theoptical waveforms output from the group delay adjuster 11A aresubstantially time-aligned with each other. The time-aligned opticalsignals on the respective channels are input to the ROSA 13 with littleor minimized time offset among the change points of the opticalwaveforms. Accordingly, influence of crosstalk among the TIA 132-0 to132-n is reduced, and electrical signals with appropriate waveforms aresupplied to the CDR 15.

FIG. 7 also illustrates an example of the group delay controlconfiguration according to the first embodiment. The group delayadjuster 11A has the group delay variable elements 112-0 to 112-nprovided on the respective channels, and propagation speed adjusters311-0 to 111-n are provided to adjust the propagation speed of the lighttravelling through the associated group delay variable elements 112-0 to112-n. When the group delay variable elements 112-0 to 112-a are madeof, for example, silicon, the index of refraction of each of the groupdelay variable elements 112-0 to 112-n is adjusted using a temperaturecontroller such as a heater.

The CDR 15 is adapted to synchronize the phases of the input signal andthe output signal using, for example, a phase locked loop (PLL) circuit151. The PLL circuit 151 has multiple circuit blocks corresponding tothe multiple channels. The number of the PLL circuit blocks is the sameas that of the multiple channels. The PLL circuit block for channel-0has a flip-flop (F/F) circuit 152-0, a phase comparator 153-0, and avoltage-controlled oscillator (VCO) 155-0.

The electrical signal output from the TIA 132-0 of channel-0 isconnected to the input of the flip-flop circuit 152-0. A portion of theinput signal to the flip-flop circuit 152-0 is branched and connected toone input of the phase comparator 153-0. The flip-flop circuit 152-0detects the edge of the input signal at the oscillation frequency of theVCO 155-0, and outputs the retimed data signal. A portion of the retimeddata signal is branched and connected to the other input of the phasecomparator 153-0.

The phase comparator 153-0 compares the phase of the signal input to thePLL 151 and the phase of the retimed data signal, and outputs a voltagerepresenting the phase difference. The phase comparator 153-0 can berealized by any appropriate configurations, for example, an XOR circuitor a flip-flop type phase frequency comparator may be used. The outputof the phase comparator 153-0 is connected to the input of the VCO155-0.

The VCO 155-0 generates a clock with a frequency and a phase accordingto the phase difference (or the voltage from the phase comparator). Theclock output from the VCO 155-0 is fed back to the flip-flop circuit152-0 and used for edge detection of the input signal. Thus, byinputting the difference between the feedback signal from the VCO 155-0and the change point (or the edge) of the electrical input signal to theVCO 155-0, the phases of the input signal and the output signal aresynchronized with each other. The clock generated by the VCO 155-0represents the change point of the electrical waveform of the inputsignal. This clock is supplied as a timing signal T0 of the channel-0 tothe controller 17.

The circuit block corresponding to channel-1 has a flip-flop circuit152-1, a phase comparator 153-1 and a VCO 155-1, and operates in thesame manner as the circuit block for the channel-0. The clock generatedby the VCO 155-1 is supplied as a channel-1 timing signal T1 to thecontroller 17.

Similarly, the circuit block corresponding to channel-n has a flip-flopcircuit 362-n, a phase comparator 153-n and a VCO 155-n, and the sameoperations as the circuit blocks of channel-0 and channel-1 areperformed. The Clock generated by the VCO 155-n is supplied as achannel-n timing signal Tn to the controller 17.

The controller 17 includes a processor 171 and a memory 172. The timingsignals T0 to Tn of the respective channels supplied from the PLLcircuit 151 are stored in the memory 172. The processor 171 generatescontrol signals to be supplied to the propagation speed adjusters 111-0to 111-n of the group delay adjuster 11A, using the timing informationstored in the memory 172.

The propagation speed adjusters 111-0 to 111-n perform temperaturecontrol or voltage control according to the control signals suppliedfrom the controller 17 to control the indexes of refraction of theassociated group delay variable elements 112-0 to 112-n, therebyadjusting the propagation speeds of the optical signals on therespective channels. This arrangement can minimize the phase or timedifference of the change points of the optical waveforms betweenchannels.

FIG. 8 is a flowchart illustrating control operations of the controller17. The controller 17 acquires the timing signals T0 to Tn representingthe clock edges of the respective channels (S11). A reference channel isselected from among the timing signals T0 to Tn (S12). The timinginformation of the reference channel is set as “Ty”. For example, timinginformation of the channel closest to the center of the channels may beset to “Ty”. Then, a difference ΔTi between the timing information “Ty”of the reference channel and the timing information of each of the otherchannels is calculated (S13). The difference ΔTi in the timinginformation is expressed by ΔTi=Ty−Ti (where “i” is an integer from 0 ton except, for the reference channel number), and it indicates the timedifference at data change point between the reference channel and any ofthe other channels.

Then, it is determined whether the difference ΔTi of each channel isgreater than zero (ΔTi>0) in S14. If ΔTi is greater than zero (Yes inS14), the timing of the target channel is delayed behind the referencechannel, and accordingly, the propagation speed of the optical signal onthat channel is increased to minimize ΔTi (S15). The propagation speedof the target channel may be increased by decreasing the index ofrefraction of the associated group delay variable element 112-i. In theexample of the control flow of FIG. 6, the associated group delayvariable element 112-i is heated for ΔT minutes to reduce the index ofrefraction. If the ΔTi is not greater than zero (No in S14), it isdetermined that the timing of the target channel is faster than thereference channel, and the index of refraction of the group delayvariable element 112-i of the target channel is increased so as tominimize the ΔTi (S16). In the example of the control flow of FIG. 6,the associated group delay variable element 112-i is cooled for ΔTminutes to increase the index of refraction, when the absolute value ofthe difference Δti is smaller than a predetermined threshold value, itmay be determined that the time amount of the group delay issubstantially the same or very close to each other in S14. In this case,the control on the index of refraction of the target channel may beskipped.

When step S15 or S16 is finished, the process returns to step S11 andrepeats the flow at the next timing. Namely, timing signals T0 to Tnobtained from the CDR 15 at the next timing are read out of the memory172, and steps S11 to S16 are repeated. The operation flow of FIG. 8 isrepeated during the service of the optical transceiver 1 to maintain theamount of group delay constant among the multiple channels.Consequently, the change points of the optical waveforms of the incomingsignals (and the change points of the electrical waveforms of the inputsignals to the CDR 15) are time-aligned among the channels, andcrosstalk can be reduced in the CDR 15.

Although in the example of FIG. 8 the channel closest to the center ofthe multiple channels is selected as the reference channel, theinvention is not limited to this example. For example, the channel withthe most advanced timing or the channel with the slowest tinting may beselected as the reference channel.

<Second Embodiment>

FIG. 9 illustrates an example of a group delay control configuration inan optical receiver 10 of the second embodiment. In the secondembodiment, bit error information acquired from an error detector isused for group delay control, and group delays of the optical waveformsare controlled among multiple channels so as to minimize the bit errorrate of each channel.

As in the first embodiment, after channel separation and beforephoto-detection by the photodiodes 131-0 to 131-n, a group delayadjuster 11A is provided on the respective channels. The group delayadjuster 11A includes group delay variable elements 112-0 to 112-nprovided for the respective channels and propagation speed adjusters111-0 to 111-n for adjusting the propagation speeds of the opticalsignals travelling through the group delay variable elements 112-0 to112-n.

The optical signals on the respective channels are detected by thephotodiodes 131-1 to 131-n. The photocurrents are converted andamplified to electric voltages by the associated TIAs. The electricalsignals are subjected to waveform shaping and retiming at the CDR 15.The CDR 15 has any suitable circuit configuration for performingextraction of clock signals and data decision, and it may use, forexample, a PLL circuit block provided for each of the channels.

The outputs of the CDR15 are converted into digital signals, subjectedto, for example, channel estimation, wavelength dispersion compensation,polarization processing, etc., and then supplied to bit error detectors19-0 to 19-n.

The bit error detectors 19-0 to 19-n detect bit error rates in theoptical signals received from the associated channels, and supply thedetection results to the controller 17. The bit error rate may be anindicator indicating signal to noise (S/N) ratio. The greater thecrosstalk between channels, the greater the bit error rate is. Thecontroller 17 controls the propagation speed adjuster 111-0 to 111-n(such as heaters) for adjusting the propagation speeds of opticalsignals travelling through the associated group delay variable elements112-0 to 112-n so as to minimize the bit error rates on the respectivechannels. The particulars of the control operations of the controller 17are described below.

FIG. 10 is a flowchart illustrating the operations performed by thecontroller 17. First, the bit error rates E0 to En of the respectivechannels detected by the bit error detectors 19-0 to 19-n are collected(S21). The value “j” which represents the sequential order of the biterror rate from the greatest to the smallest is initialized (j=1), where“j” is an integer from 1 to n+1 (S22). After initialization of the valuej, the channel with the greatest bit error rate is selected for thefirst round, then the channel with the j-th greatest bit error rate isselected from E0 to En, and the bit error rate of the selected channelis set to Ex (S23). A control routine of group delay adjustment isperformed on the group delay variable element 112-x of the selectedchannel (S24). when the control routine for the selected channel isfinished, it is determined whether the group delay adjustment has beenmade for all the channels, i.e., whether the value “j” has reached j=n+1(S25). If there are any channels left unprocessed (No in S25), the value“j” is incremented to j=j+1 (S26), and the process returns to step S23.In S23, the next channel with the second largest (for the second round)or the (j+1)-th largest (for the (j+1)-th round) is selected and the biterror rate of the selected channel is set to Ex. Then, the controlroutine of the group delay adjustment is performed on that channel(S24). Steps S23 to S25 are repeated until all the channels have beenprocessed. When the group delay adjustment is completed for all thechannels (Yes in S25), the process returns to S21 and steps S21 to S26are performed for the subsequently collected data set.

This control process is performed repeatedly during the service of theoptical transceiver 1 to maintain the quantities of the group delaysconstant among the multiple channels. With this arrangement, the changepoints of the electrical waveforms to be input to the CDR 15 aretime-aligned between channels and crosstalk can be reduced.

FIG. 11 is a flowchart illustrating a particular example of the controlroutine (S24) of FIG. 10 performed on the group delay variable elements.In FIG. 11, it is assumed that the group delay variable elements 112-0to 112-n are made of, for example, silicon. First, silicon is heatedwith a certain step size until the silicon temperature rises bypredetermined degrees (S31). The step size of the heating nay be set toan appropriate value, and in this particular example of FIG. 11, it isset to 5° C.

After the heating, the bit error rate of the focused-on channel isacquired from the associated bit error detector 19-x and the acquiredbit error rate is set to Ex1 (S32). Then, it is determined whether thebit error rate Ex1 after the heating becomes smaller than the bit errorrate Ex before the heating (Ex1<Ex) (S33). If the bit error rate Ex1 isreduced by the heating (Yes in S33), it means that the direction ofcontrol is correct. In this case, the group delay variable element ofsilicon is further heated to raise the temperature by 5° C. (S34), andbit error rate Ex2 after the heating is acquired from the same bit errordetector 19-x (S35). It is again determined whether the newly acquiredbit error rate Ex2 has exceeded the previous bit error rate Ex1(Ex2>Ex1) (S35). If the new bit error rate Ex2 is equal to or less thanthe previous error rate Ex1 (No in S36), the direction of control isstill correct. In this case, the bit error rate of this channel isupdated from Ex1 to Ex2 (Ex1=Ex2) (S37), and the steps S34 to S36 arerepeated. If the newly acquired bit error rate Ex2 becomes greater thanthe previous error rate Ex1 (Yes in S36), the silicon is overheated.Accordingly, the group delay variable element is cooled by 5° C. and thecontrol routine for this channel is finished. Then the process returnsto step S25 of FIG. 10.

If in step S33 Ex1 does not become smaller than Ex (No in S33), it meansthat the direction of control is incorrect, and the process jumps tostep S39 and the control is performed in the reverse direction. Namely,the group delay variable element of silicon is cooled by 5° C. (S39),and the bit error rate Ex2 after the cooling is acquired from the biterror detector 19-x (S41). It is then determined whether the newlyacquired bit error rate Ex2 has become greater than the previous biterror rate Ex1 (Ex2>Ex1) (S42). If the new bit error rate Ex2 is equalto or leas than the previous error rate Ex1 (No in S42), the directionof control is correct. In this case, the acquired bit error rate isupdated from Ex1 to Ex2 (Ex1=Ex2) (S43), and then steps S39 to S42 arerepeated. If the new bit error rate Ex2 has become greater than theprevious bit error rate Ex1 (Yes in S42), the silicon is overcooled.Accordingly, the group delay variable element is heated by 5° C. (S44),and the control routine for this channel is finished. Then, the processreturns to step S25 of FIG. 10.

With this method, the bit error rate becomes the minimum on each channeland the S/N ratio is improved. This means that crosstalk betweenchannels is reduced. The same control operations apply to the groupdelay variable elements 112-0 to 112-n made of an electro-optic materialor a photonic crystal. The voltage level applied to the electro-optic orphotonic crystal is changed with a predetermined step size so as tominimize the bit error rate of each of the multiple channels, and theinfluence of inter-channel crosstalk can be reduced.

The sequential order of the process for minimizing the bit error rate isnot limited to the descending order of the bit error rate from thegreatest to the smallest. The process may be performed in the numericalorder of the channel index.

FIG. 12 illustrates one of the advantageous effects of the embodiments.In the first and second embodiments described above, crosstalk isreduced by bringing the data change points of the optical waveforms ofthe incoming signals to be in time-alignment between all the channels.Besides, degradation in receiver sensitivity is also avoided if asufficient amount of margin is assured for threshold decision at thephase of decision.

The worst condition that causes degradation in receiver sensitivity isthe case in which the optical waveform of one channel changes greatly atthe timing (or the phase) of data decision. In this worst case, theerror race in data decision (e.g., bit “0” or bit “1”) is likely toincrease due to the decreased margin for the decision threshold. Incontrast, at the optimum timing, the data change points of the opticalwaveforms are time-aligned among channels. Even if it is difficult tocorrect the data change point of the optical waveform into the optimumtiming (or phase) depending on limitation of materials or other reasons,the accuracy of data decision can be maintained as long as the worsecondition of receiver sensitivity degradation is avoided. Thus, bycontrolling the group delay in the direction where the change point ofthe received optical waveform of one channel is away from the datadecision timing of the other channels, the influence of crosstalk can bereduced.

The embodiments described above are exemplary and many othermodifications and alterations are included in the invention. Forexample, the receiver-side CDR 15 and the transmitter-side CDR 25 may beintegrated on a single chip in the optical transceiver 1. A CPU commonlyused in the optical transceiver 10 and the optical transmitter 20 may beused as the controller 17. Multi-channel transmission may be implementedby space division (or mode division) multiplexing, in place of thewavelength division multiplexing. As a photodetector, any typo ofphotodiode such as a PN photodiode, PIN photodiode, avalanchephotodiode, etc., capable of outputting photocurrent in proportion tothe incident light may be used.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of superiority orinferiority of the invention. Although the embodiments of the presentinventions have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. An optical receiver comprising: photodetectorseach provided on a corresponding channel of multiple channels andconfigured to detect an optical signal at the corresponding channel, adelay adjuster circuit provided before the photodetectors and configuredto adjust a delay time of an optical waveform of an incoming signal onat least one channel, and a processor that controls the delay time suchthat a change point of a first optical waveform of a first channel isaway from a data decision timing of a second optical waveform of asecond channel, using information acquired from an electrical signalproduced after photo-detection, wherein the processor receives a biterror rate of each of the multiple channels and controls the delay timein the delay adjuster circuit so as to minimize the bit error rate ofsaid each of the multiple channels.
 2. The optical receiver as claimedin claim 1, wherein the processor controls the delay time so as to bringthe change point of the first optical waveform of the first channel tobe in time-alignment with a change point of the second optical waveformof the second channel.
 3. The optical receiver as claimed in claim 1,wherein the delay adjuster circuit has, on each of the multiplechannels, a group delay variable element made of silicon, a polymer, anelectro-optic material, or a photonic crystal, and a propagation speedadjuster that changes an index of refraction of the group delay variableelement to adjust a propagation speed of the optical signal passingthrough the group delay variable element.
 4. The optical receiver asclaimed in claim 1, further comprising: a clock data recovery circuitprovided behind a block of the photodetectors and configured to generatetiming signals for data decision on the multiple channels, wherein theprocessor controls the delay time in the delay adjustor circuitaccording to the timing signal.
 5. The optical receiver as claimed inclaim 4, wherein the processor selects a reference channel according tothe timing signals and controls the delay time in the delay adjustercircuit so as to minimize a time difference between the data decisiontiming of the reference channel and the data decision timing of theother channels.
 6. An optical receiver comprising: photodetectors eachprovided on a corresponding channel of multiple channels and configuredto detect an optical signal at the corresponding channel, a delayadjuster circuit provided before the photodetectors and configured toadjust a delay time of an optical waveform of an incoming signal on atleast one channel, and a processor that controls the delay time suchthat a change point of a first optical waveform of a first channel isaway from a data decision timing of a second optical waveform of asecond channel, using information acquired from an electrical signalproduced after photo-detection, wherein the processor controls the delaytime so as to bring the change point of the first optical waveform ofthe first channel to be in time-alignment with a change point of thesecond optical waveform of the second channel, and wherein the processorreceives signal reception quality information of each of the multiplechannels and controls the delay time in the delay adjuster circuit basedon the signal reception quality information.
 7. The optical receiver asclaimed in claim 1, further comprising: amplifiers each provided on acorresponding channel of the multiple channels and configured to producean electric voltage signal from an output of a correspondingphotodetector of the photodetectors, wherein the amplifiers of themultiple channels are integrated in a single unit.
 8. An opticaltransceiver comprising: the optical receiver as claimed in claim 1; andan optical transmitter.
 9. A control method of reception of opticalsignals, comprising: receiving at an optical receiver an optical signalon each of multiple channels; detecting the optical signal and producingan electrical signal for each of the multiple channels; and based uponinformation acquired from the electrical signal, controlling a delaytime of an optical waveform of an incoming signal for each of themultiple channels such that a change point of a first optical waveformof a first channel is away from a data decision timing of a secondoptical waveform of a second channel, wherein a bit error rate of eachof the multiple channels is received and the delay time is controlled soas to minimize the bit error rate of said each of the multiple channels.10. The control method as claimed in claim 9, wherein the controlling ofthe delay time is performed on the optical waveform to be input to aphotodetector that detects the incoming signal on said each of themultiple channels such that the change point of the first opticalwaveform of the first channel is time-aligned with a change point of thesecond optical waveform of the second channel.
 11. The control method asclaimed in claim 9, further comprising: producing a timing signal fordata decision from the electrical signal for each of the multiplechannels; and controlling the delay time of the optical waveform basedupon the timing signal for each of the multiple channels.
 12. A controlmethod of reception of optical signals, comprising: receiving at anoptical receiver an optical signal on each of multiple channels;detecting the optical signal and producing an electrical signal for eachof the multiple channels; based upon information acquired from theelectrical signal, controlling a delay time of an optical waveform of anincoming signal for each of the multiple channels such that a changepoint of a first optical waveform of a first channel is away from a datadecision timing of a second optical waveform of a second channel, andacquiring information representing a signal reception quality from theelectrical signal for each of the multiple channels, wherein the delaytime of the optical waveform of the incoming signal is controlled basedupon the signal reception quality, and wherein the controlling of thedelay time is performed on the optical waveform to be input to aphotodetector that detects the incoming signal on said each of themultiple channels such that the change point of the first opticalwaveform of the first channel is time-aligned with a change point of thesecond optical waveform of the second channel.
 13. An opticaltransceiver comprising: the optical receiver as claimed in claim 6; andan optical transmitter.